Inkjet printing mechanisms use cartridges or "pens" which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles or slits through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis. For storage, or during non-printing periods, service stations usually include a capping system which seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through (each of the nozzles in a process known as "spitting." Typically, the waste ink is collected in a stationary reservoir portion of the service station, which is often referred to as a "spittoon." After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
Early inkjet printers used a single monochromatic pen, typically carrying black ink. Later generations of inkjet printing mechanisms used a black pen which was interchangeable with a tri-color pen, typically one carrying the colors of cyan, magenta and yellow within a single cartridge. The tri-color pen was capable of printing a "process" or "composite" black image, by depositing a drop of cyan, a drop of magenta, and a drop of yellow ink all at the same location. Two of these earlier single-pen, interchangeable inkjet printing mechanisms were sold by the Hewlett-Packard Company of Palo Alto, Calif., the present assignee, as the DeskJet.RTM. 310 portable inkjet printer and the DeskJet.RTM. 400 desktop inkjet printer. Unfortunately, images printed with the composite black usually had rough edges, and the overall image, even the color portions, often had a non-black hue or cast, depending for instance, upon the type of paper used.
The next generation of printers further enhanced the images by using a dual pen system. These dual pen printers provided a black pen along with a tri-color pen, both of which were mounted in a single carriage. These dual pen devices had the ability to print crisp, clear black text while providing full color images. One earlier dual pen inkjet printing mechanism was sold by the Hewlett-Packard Company of Palo Alto, Calif., the present assignee, as the DeskJet.RTM. 320 portable inkjet printer. As another answer to the dissatisfaction with the composite black images, a quad pen printing mechanism was developed which carried four cartridges in a single carriage. These quad pen printing mechanisms had a first pen carrying black ink, a second pen carrying cyan ink, a third pen carrying magenta ink, and a fourth pen carrying yellow ink.
Unfortunately, both the quad pen printers and the dual pen printers produced images, such as photographic images, which had a "grainy" appearance. For example, when printing a light colored portion of an image, such as a flesh tone, yellow dots were printed and lightly interspersed with magenta dots. When viewed at a distance, these magenta dots provided a flesh tone appearance; however, upon closer inspection the magenta dots were quite visible, giving the image an undesirable grainy appearance. This grainy appearance was similar to the graininess seen in newspaper photographs, or in photos taken using the wrong speed ("ASA" or "ISO" rating) of photographic film in low light conditions. Inkjet printing mechanisms are known as "binary drop devices" because they form images either by firing to place a drop of ink on the print medium, or by not firing. Not firing a droplet leaves either the print medium, or a previously printed drop(s), exposed to view. Unfortunately, such binary drop devices give inherently grainy images due to the visual "step" between the "drop on" and "drop off" regions. Worse yet, the larger the drops printed, the more grainy the resulting image appears, whether printing color or gray-scale images.
These earlier inkjet printers provided crisp black text and bright vivid graphics and charts, yet they failed to provide images of near photographic type quality, such as portrait, scenic landscapes, and other natural appearing images. Other devices have been used to provide high quality images, such as continuous tone devices some of which use a dye sublimation processes. Unfortunately, these continuous tone devices are expensive, and very unlikely to be viable within the small office and home printer markets, which currently sell printers to consumers within the price range of $200-$1,000 dollars.
Another printing, system, known as an "imaging" printing system, has been proposed. Using a basic dual pen printer platform, typically constructed for a monochrome (e.g. black) cartridge and a tri-color (e.g. cyan, magenta, yellow) cartridge, the monochrome cartridge is replaced with a tri-chamber "imaging cartridge." While the normally installed tri-color cartridge carries full colorant concentrations of inks, the imaging cartridge typically carries ink formulations having reduced colorant concentrations. For instance, the imaging cartridge may contain reduced colorant concentrations of cyan and magenta, and a full concentration of black ink. By interspersing droplets of reduced colorant concentration with droplets of the full colorant concentrations, the resulting images have a near photographic quality.
Unfortunately, in a dual pen inkjet printer, this ability to interchange the monochrome and multi-cartridges presents a unique set of problems when it comes to servicing of both types of cartridges. One earlier imaging inkjet printing mechanism was the model 693C DeskJet.RTM. inkjet printer sold by the Elewlett-Packard Company of Palo Alto, Calif., the present assignee. This system used dye-based color inks and a pigment-based back ink, which had different servicing needs than the dye-based color inks. In this earlier imaging system, a single, enlarged cap with multiple sealing lips was used to seal either the black pen or the imaging pen when installed. A single wiper was also used to service either the black pen or the imaging pen, with the wiper being of a more rigid upright profile, mounted on a spring-loaded arm to avoid excessive wiping forces which may otherwise damage the printhead. In transitioning to a completely dye-based ink imaging system, the fear of contamination between a dye-based black ink and reduced colorant concentrations of dye-based imaging inks lead to the rejection of a servicing system which used the same set of caps and wipers for both pens. Any contamination with back ink of the reduced dye loads of magenta, and particularly yellow, could seriously degrade the print quality of the resulting image. Thus, the service station for a dye.-based ink imaging system must accommodate the servicing needs of the monochrome black and imaging cartridges, without contaminating one pen with residue from the other pen.